Viscoplastic creep and microstructure evolution of Sn-based lead-free solders at low strain

In this study, the viscoplastic creep behaviors of the Sn-4Ag/Cu, Sn-3Cu/Cu and Sn-58Bi/Cu solder joints under shear stress were in-situ observed, and the evolutions in microstructure of the solders were characterized by in-situ EBSD. The results reveal that the creep strain of the Sn-Cu and Sn-Ag solders increase linearly with increasing time. Deformation of the Sn-Cu solder concentrates in some parallel shear bands, while deformation of the Sn-Ag solder is relatively uniform, and the deformation concentration along the joint interface is not obvious for the two solders. The strain concentration around the fine Ag3Sn particles in the Sn-Ag solder is not obvious, while the strain concentration around the large Cu6Sn5 grain in the Sn-Cu solder is serious and result in fracture of the Cu6Sn5 grain. Dynamic high-temperature recovery occurs in the Sn-Ag and Sn-Cu solders, result in grain boundary migration, polygonization and formation of a few low-angle grain boundaries. The Sn-Bi solder deforms through grain boundary sliding and phase boundary sliding, the strain increase exponentially with increasing time, and its grain structure is stable during the deformation process.